The present invention is related to drive means for positioning tables and, more particularly, to drive means for XYZ positioning tables wherein at least one table is constrained to travel to controllable positions along a longitudinal axis.
Positioning tables requiring high accuracy, such as, for example, positioning tables used in the preparation of solid state components and the like, require positioning accuracy measured in units on the order of 0.0001 inch. The attainment of such accuracy does not permit ignoring any sources of error which can be eliminated.
A positioning apparatus for one axis conventionally employs a table which is longitudinally guided along machined grooves in a frame into which rolling or sliding members on the table are fitted. A lead screw, rotated by a servomotor extends along a center line of the entire path to be traversed by the table. Traveling nuts affixed to the underside of the table engage the lead screw and, as the servomotor turns clockwise and counterclockwise, move the table back and forth. Optical or other conventional means affixed to the table are employed to measure the table position along its axis and to produce signals which, when fed to a conventional table controller, are employed to generate control signals for application to the servomotor.
In very precise applications, position inaccuracies arise from a number of sources. If the guides which constrain the longitudinal motion of the table are not accurately parallel to the lead screw, the preloading between the table and the guides varies from position to position along the travel and consequently produces a positioning error. Furthermore, lack of concentricity in the servomotor bearing can also produce error. Also, a certain amount of backlash can be expected in a screw-driven apparatus.
In addition to high accuracy, some applications require rapid motion from position to position. Positioning tables operating under start-stop-reverse conditions require high torque for achieving high positioning rates. The inclusion of a relatively massive lead screw in prior art systems adds to the torque requirement placed on the servomotor and/or limits the maximum positioning rate.